In some injection molding apparatuses, a valve pin is movable in a melt channel in a manifold block to control melt flow in the melt channel. A portion of the valve pin extends into the melt channel to control the melt flow, and a portion extends outside the melt channel for connection to an actuating mechanism, which is typically contained in an actuator block.
The valve pin passes through a pass-through in a valve bushing, which is outside the melt channel. The valve bushing guides the movement of the valve pin and seals against melt leakage out of the melt channel. The valve bushing also serves as a spacer between the manifold block and the actuator block, to thermally insulate the manifold block from the actuator block. Insulating the manifold block facilitates controlling the temperature of the manifold block and the melt channels contained therein.
Several types of valve bushings have been developed over the years. For example, U.S. Pat. No. 5,022,846 (Schmidt) discloses a valve bushing that includes a body portion having a pass-through for the valve pin, and a spacer portion for spacing the manifold block from the actuator block. Several problems exist with the valve bushing of Schmidt. The pass-through typically wears over time from the cycling of the valve pin, and eventually, the gap between the pin and the hole permits the leakage of melt therethrough. Once the leakage is excessive, the entire bushing must be replaced. Furthermore, the insulating effect caused by the spacing of the manifold block from the actuator block is partially lost since the valve bushing itself acts as a thermal conductor and conducts heat from the manifold into the actuator block.
U.S. Pat. No. 5,955,121 (Gellert et al.) discloses a valve bushing assembly composed of two pieces. The valve bushing has an insert and a sealing bushing. The insert includes a spacer portion for spacing the manifold block from the actuator block. Both the insert and the sealing bushing seal around the valve pin, and are thus, both subject to wear from the cycling of the valve pin. Furthermore, the insert acts as a thermal conductor between the manifold block and the actuator block, reducing the efficacy of any temperature control system for the manifold.
Another issue with valve bushings of the prior art is the need to provide an effective seal, both between the bushing and the valve pin, and also between the bushing and the manifold block.
There is a continuing need for new valve bushing systems for guiding a valve pin and for spacing between blocks in an injection molding apparatus.